The present invention relates to a stage having a movable mirror attached thereto, for carrying a material piece such as a reticle or a wafer and used in a projection exposure apparatus, for example, and also relates to a supporting mechanism for supporting such a movable mirror on such a stage. More particularly, the present invention relates to a stage having a movable mirror attached there to, the movable mirror forming a part of an interferometric measurement system for measuring the position of the stage, and also relates to a supporting mechanism for supporting such movable mirror on such a stage.
In a lithographic process used to fabricate semiconductor devices or the like, there have been commonly used demagnification projection exposure apparatus of the type in which a wafer undergoes stepping movements for sequential exposure operations to a plurality of exposure sites on the wafer (the exposure apparatus of this type are called "steppers"). The apparatus has a moving stage for carrying a material piece such as a wafer and capable of two-axis translational movements in a plane (or "XY-stage"). Further, in order to measure X- and Y-coordinate positions of the moving stage, a pair of light wave interferometric measurement systems are commonly used as an apparatus for measuring the coordinate positions (distances) by utilizing the interference between two coherent light beams such as laser beams.
Generally, the apparatus of this type comprises a reflecting mirror (or movable mirror) which is mounted on one end of the moving stage (which is the measuring object) such that it extends along one side edge of the moving stage and in a direction normal to the measuring direction. The apparatus further comprises an interferometer which is disposed outside the moving stage so as to face the mirror surface of the movable mirror. The interferometer emits a beam of frequency-stabilized laser (such as helium-neon laser) to the movable mirror so as to provide interference between the beam reflected by the movable mirror and another beam not reflected by the movable mirror, which interference is used to achieve very precise measurement with a high resolution of about 0.01 .mu.m, for example. The interferometer cannot operate without the moving mirror provided on the measuring object (or moving stage), and so the movable mirror forms part of the interferometric measurement system.
FIG. 1 schematically shows an exemplified supporting mechanism for supporting a movable mirror on a moving stage, which may be used for the apparatus of this type. As is common in the art, a reflecting mirror or movable mirror 1 (shown by imaginary lines in FIG. 1) is formed by a glass body having a rectangular cross-section with four side surfaces, one of which is silvered to form a mirror surface. The movable mirror 1 is supported in the vertical direction by a pair of movable mirror supporting surfaces 4A and 4B, which comprise top surfaces of respective raised portions formed on a top surface of an elongate, belt-like raised portion 3 having a predetermined width. The raised portion 3 is formed on one side edge of the stage body 2, and the movable mirror supporting surfaces 4A and 4B are spaced apart from each other in the longitudinal direction of the raised portion 3. The movable mirror 1 is further supported in the measuring direction (represented by arrow A in FIG. 1) such that it is clamped between i) a pair of supporting surfaces 6A and 6B and ii) an associated pair of preloaded push pins 9A and 9B. The supporting surfaces 6A and 6B are vertical, rectangular surfaces on the inside of corresponding pair of reference protrusions 5A and 5B having a box-like shape and formed on the top surface of the stage body 2 at positions adjacent to the movable mirror supporting surfaces 4A and 4B, respectively, and on one side of these surfaces 4A and 4B in the measuring direction. The push pins 9A and 9B are slidably supported by corresponding pair of push-pin-supporting protrusions 7A and 7B formed on the top surface of the stage body 2 at positions adjacent to the movable mirror supporting surfaces 4A and 4B, respectively, and on the other side of these surfaces 4A and 4B in the measuring direction. The push pins 9A and 9B are inserted from the outside of the push-pin-supporting protrusions 7A and 7B into corresponding holes formed in the protrusions 7A and 7B, respectively, and are urged by respective, associated coil springs 8A and 8B toward the supporting surfaces 6A and 6B, respectively. The push pins 9A and 9B have flat tip end surfaces.
Unfortunately, this conventional movable mirror supporting mechanism suffers from a problem that when the stage body 2 supporting the reflecting mirror or movable mirror 1 is driven in a certain direction (e.g. in the measuring direction), an inertial force produced by the acceleration of the driven stage body 2 acts on the moving mirror 1 which is clamped between i) the supporting surfaces 6A and 6B of the reference protrusions 5A and 5B and ii) the preloaded push pins 9A and 9B facing to the supporting surfaces 6A and 6B, resulting in that the coil springs 8A and 8B may possibly yield or be compressed to allow the movable mirror 1 to displace apart from the supporting surfaces 6A and 6B into an erroneous position and, in certain circumstances, even remain in that erroneous position. This results in that the position (translational or angular) of the movable mirror 1 relative to the stage body 2 may possibly change while the stage body 2 is being driven, and that a considerable error may possibly occur in the measured value provided by the interferometric measurement system.
In view of the foregoing, it could be one approach to this problem that, instead of clamping the moving mirror 1 by urging it against the supporting surfaces 6A and 6B by the preloaded push pins 9A and 9B, the moving mirror 1 is fixedly secured onto the stage body 2, so that any displacement of the movable mirror 1 relative to the stage body 2 may be prevented even when an inertial force produced by the acceleration of the driven stage body 2 acts on the movable mirror 1.
However, as commonly known to those skilled in the art, the glass body constituting the movable mirror 1 cannot be rigid enough to be considered as an ideal rigid body; in fact, various forces acting on such movable mirror 1 would cause minute but nonnegligible deformations of the moving mirror 1. In addition, any deformation of the mirror surface of the movable mirror 1 which may be caused by the supporting arrangement may greatly affect the measuring accuracy of the interferometric measurement system because the resolution of the measurement is very high (about 0.01.mu..mu.). As the result, the arrangement in which a movable mirror is fixedly secured onto a stage body by means of fixtures has not been used so far.
Regarding the materials for a stage body or material piece support and a movable mirror or reflecting mirror for use with an interferometer and supported on the stage body, there have been three major options. The first option is that the movable mirror is made of an optical glass while the stage body is made of a ceramic material. The second option is that either of them is made of an optical glass. The third option is that both of them are made of a ceramic material.
Unfortunately, with a conventional stage or a material piece support comprising a movable mirror made of an optical glass and a stage body made of a ceramic material, the movable mirror may be subject to deformation caused by a temperature change, since the movable mirror and the stage body are made of different materials so that they have different thermal expansion coefficients. That is, when the temperature in the environment of the movable mirror changes, the movable mirror and the stage body produce different thermal expansions, resulting in some harmful deformation of the mirror surface of the movable mirror, which leads to a deterioration in the accuracy of the measured positions of the stage body determined by the interferometer.
In the case where the movable mirror and the stage body are made of an optical glass or optical glasses, the stage body should have a less rigidity and necessitate a higher cost than a stage body made of a ceramic material. The less rigidity of the stage body results in the less stability in positioning of the mirror surface of the movable mirror carried on the stage body, leading to a possible deterioration of the measuring accuracy of the interferometer.
In the case where the movable mirror and the stage body are made of a ceramic material, there arise a problem that the reflectivity of the movable mirror is lowered by the existence of pores in the ceramic material. In general, powder which is to be sintered into a certain ceramic material has a porosity of about 40%. Through the sintering process, the porosity lowers due to various factors including the surface tension of molten particles. Thus, most of the space contributing the porosity vanishes during the sintering process, and a little space remains after the sintering process and forms pores in the ceramic material. Apparently, a higher porosity of the ceramic material for the movable mirror results in a lower reflectivity of the movable mirror. The porosity of a ceramic material may be reduced by suitably selecting the powder and sintering method used for the ceramic material, and thereby a more compact ceramic material may be obtained; however, in general, a highly compact ceramic material created in this manner is very costly. Accordingly, when the stage body and the movable mirror were formed as an integral part made of a certain, low-porosity, compact ceramic material, there would arise a problem that a large amount of such costly ceramic material is required, which increases the total cost of the stage.